I paid 95k for it.Pending the outcome of a CMA the potential equity from sale would go a long way to getting me into a fix and flip.

At that point, it may not matter what kind of contract or what agreements are made but more to the outcome expected from doing multiple transactions.

What do you think would have been the outcome if you did not agree to selling the house back to the  previous owner. 

An option, call or put, can be used as a standalone contract or in various combinations simultaneously to bet on any potential outcome.

Behavioral studies show that about 65 to 70 percent of human motivation comes from avoiding negative consequences, while only 30 to 35 percent comes from seeking positive outcomes.

I had the same desire as well, you can can read my story at this thread: I quit my CPA Job to Buy Large Apartments.You stated above of your goal:I too want to buy a multi-family / apartment complex in the USA.Now really rip this goal apart:WHY do you want to buy MFHow many unitsWhat type of deal -momentum, value-add, repositioningHow much money do you NEED to make buying MFHow much do you WANT to make buying MFWhere are the properties located - own backyard/different stateDo you have time to be involved in this businessWho will manageHow will you find the dealsDo you have the funds to closeWill you do a syndicationWho is on your team - brokers, cpa, attorney, contractorI can keep going...The purpose of this exercise is to get you very clear on your specific outcome.

After a periods of many months we learned the outcome was this: one note was a grand slam, one was a home-run, and two we didn't make any money on.

Not the numbers I would have hoped for, and we definitely need to improve our rehab estimates and/lower costs, but I'm still pretty happy with the outcomes.  

I would be interested in hearing what the outcome or plan of attack ends up being if an attorney is engaged.

While a promissory note can seem like an easy DIY, if neither of you have lent money before and may not be sure of the array of potential outcomes which do not include everything going as planned, then it is probably best to seek an attorney to help draft the documents for the loan.